Anomaly detection using an antenna baseline constraint

ABSTRACT

Systems and methods are provided for monitoring carrier phase anomalies in a range finding system. A relative carrier phase between first and second antennas is predicted as a function of a relative position between the two antennas. A relative carrier phase between the first and second receivers is measured based upon at least one transmitted signal received at each of the first and second antennas. An anomaly detection metric is calculated as a difference between the measured relative carrier phase and the predicted relative carrier phase. It is then determined if an anomaly is present according to the anomaly detection metric.

FIELD OF THE INVENTION

The invention relates generally to range finding systems, which caninclude Global Navigation Satellite Systems (GNSS) Receivers. Morespecifically, the invention relates to anomaly detection in a multipleantenna system that utilizes a known or measured baseline betweenantennas.

BACKGROUND OF THE INVENTION

In range finding applications, a receiver can utilize informationextracted from messages received from one or more transmitters todetermine the transit time of each message. A distance to eachtransmitter can be determined from the transit time given the knownpropagation speed of electromagnetic radiation, and a position of thereceiver, at least relative to the transmitters, can be determined viamulti-lateration. A well-known example of a range finding application isthe Global Positioning System.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, a method isprovided for monitoring carrier phase anomalies in a range findingsystem. A relative carrier phase between first and second antennas ispredicted as a function of a relative position between the two antennas.A relative carrier phase between the first and second receivers ismeasured based upon at least one transmitted signal received at each ofthe first and second antennas. An anomaly detection metric is calculatedas a difference between the measured relative carrier phase and thepredicted relative carrier phase. It is then determined if an anomaly ispresent according to the anomaly detection metric.

In accordance with another aspect of the present invention, a systemincludes a first antenna configured to receive a signal from atransmitter and a second antenna configured to receive the signal fromthe transmitter, with the second antenna being separated from the firstantenna by a baseline. A signal processor is configured to calculate ameasured relative carrier phase between the first antenna and the secondantenna according to the received signal. A relative carrier phaseestimator is configured to estimate a predicted relative carrier phasebetween the first antenna and the second antenna according to thebaseline between the first antenna and the second antenna. An anomalydetection component is configured to determine if an anomaly is presentaccording to an anomaly detection metric. The anomaly detection metricis determined as a function of a difference between the measuredrelative carrier phase and the predicted relative carrier phase.

In accordance with still another aspect of the present invention, aglobal navigation satellite system includes a first receiver configuredto receive signals from a plurality of GNSS satellites and a secondreceiver configured to receive signals from the plurality of GNSSsatellites, with respective antennas of the first and second receiversbeing separated by a known baseline. A signal processor is configured tocalculate a double differenced carrier phase between the first receiverand the second receiver according to the received GNSS satellitesignals. A relative carrier phase estimator is configured to estimate apredicted relative carrier phase between the first receiver and thesecond receiver according to the baseline between the first antenna andthe second antenna. An anomaly detection component is configured todetermine that an anomaly is present if a difference between themeasured relative carrier phase and the predicted relative carrier phaseexceeds a predetermined threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, wherein:

FIG. 1 illustrates one example of a range finding system utilizinganomaly detection in accordance with an aspect of the present invention;

FIG. 2 illustrates one implementation of a global navigation satellitesystem utilizing anomaly detection in accordance with an aspect of thepresent invention;

FIG. 3 illustrates a controlled reception pattern antenna configured toutilize an anomaly detection system in accordance with an aspect of thepresent invention;

FIG. 4 illustrates a redundant antenna arrangement to facilitate the useof an anomaly monitoring system in accordance with an aspect of thepresent invention; and

FIG. 5 illustrates a method for monitoring carrier phase anomalies in arange finding system in accordance with an aspect of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with an aspect of the present invention, systems andmethods are provided for anomaly detection in range finding systems. Thesystem deals with detection of measurement anomalies by using knowledgeof the relative distance between the receivers to predict some aspectsof the expected measurements. The systems and methods described hereinidentify measurement anomalies by applying a known baseline constraintto the incoming carrier phase measurements between two antennas topredict the measurements and detect deviation from the predictedmeasurements. The incoming relative carrier phase measurements arepredicted using precise knowledge of the position difference betweenthese two antennas, either in space or in time. This measurementprediction provides a reference for detecting carrier phase anomaliesaffecting either of the receivers in the baseline. This technique is notdependent on statistically resolving carrier cycle counts since they aredirectly computed at each time epoch. Precise knowledge of the baselinecan either come from a priori knowledge of a rigid baseline (e.g.,antenna self-calibration survey) or from a secondary measurement of theflexible antenna baseline (e.g., laser ranging).

Carrier phase measurements are very precise due to a receiver's abilityto track the carrier within a small fraction of its wavelength. Carrierphase measurements are used in many different domains including globalnavigation satellite systems (GNSS) such as the Global PositioningSystem (GPS) interferometry and Very Long Baseline Interferometry (VLBI)radar processing. The measurement consumer in each domain desires anassessment of the measurement accuracy and reliability. Systems andmethods in accordance with an aspect of the present invention provide analgorithm for receiver-level anomaly detection in any domain that usescarrier phase interferometry by comparing the received measurements withsynthetic measurement predictions. In addition, this technique can beused to provide geometrically constrained corrections forpost-correlation digital beam forming by predicting the differentialcarrier phase measurements produced by a controlled reception patternantenna (CRPA) in conjunction with the inertial attitude and headingmeasurements.

FIG. 1 illustrates one example of a range finding system 10 utilizinganomaly detection in accordance with an aspect of the present invention.The range finding system 10 includes at least two antenna locations 12and 14 associated with at least one antenna configured to receivesignals from one or more transmitters. In general, the system will havemultiple antennas, but it will be appreciated that the antenna locations12 and 14 can represent multiple measurements at a single antenna madeat two different times on a moving platform. Where multiple antennas arepresent, each antenna will generally be associated with a specificreceiver platform, but it will be appreciated that in some applications,a single platform may have multiple, and even redundant, antennas toallow for integrity monitoring of the signals received at the antennas.Each of these signals will generally contain a structured orpseudo-random code that can be used for determining a time-of-flight forthe signal. The received signals are processed at an associated signalprocessor 16 that calculates a measured relative carrier phase between agiven pair of the at least two antenna locations 12 and 14. In oneimplementation, the measured relative carrier phase is determined as adouble differenced relative carrier phase.

In accordance with an aspect of the present invention, the given pair ofthe at least two antenna locations 12 and 14 can be separated by a knownbaseline 18. In one implementation, the baseline 18 is fixed, such thata measurement prior to operation of the system can be used. The carrierphase ambiguity can be resolved using the fixed baseline constraint. Forexample, the baseline length can be surveyed in a body frame associatedwith the system 10 using external sensors and then transformed from bodycoordinate frame to navigation coordinates. If the platform for thesystem is nominally a two-dimensional platform such as a car or train,only a heading is required from an external sensor, such as amagnetometer, for this transformation. For a three-dimensional platformsuch as a boat or aircraft, a leveled inertial measurement can be addedto estimate the roll and pitch of the platform to allow for the shift inthe orientation of the baseline relative to the navigation frame ofreference to be tracked. As long as the external sensors provide anaccurate enough orientation to predict the rotated baseline withinone-half of a wavelength of the range finding system 10, the ambiguitycan be deterministically resolved in a single epoch.

It will be appreciated that the baseline 18 can be flexible, that is,variable, and in these implementations, the baseline is monitored on aperiodic or continuous basis by an external measurement device (notshown). For example, the two antenna locations 12 and 14 can be ondifferent mobile platforms or located on a portion of a single platformthat flexes with movement, such that their relative positions are notfixed. In one implementation, laser ranging can be used to monitor theflexible baseline. A relative carrier phase (RCP) estimator 22 isconfigured to calculate a predicted relative carrier phase from theknown baseline. In one implementation, the predictive relative carrierphase is calculated as a function of the known baseline and arepresentation of a line of sight between the one or more transmittersand at least one of a first antenna location of the given pair 12, asecond antenna location of the pair 14, and a point located on thebaseline 18 between the two antenna locations.

Each of the measured carrier phase and the predicted relative carrierphase are provided to an anomaly detection component 24. The anomalydetection component 24 determines if a measurement anomaly is presentaccording to an anomaly detection metric determined as a function of adifference between the predicted relative carrier phase and the measuredrelative carrier phase. For example, the anomaly detection component 24can provide the calculated anomaly detection metric to as a feature toan expert system, for example, a regression model, an artificial neuralnetwork classifier, or a rule-based expert system, to determine if ananomaly is present. It will be appreciated that multiple anomalydetection metrics can be accumulated over a predetermined period oftime, such that an expert system can utilize a time series of metrics asclassification features. In one implementation, the anomaly detectioncomponent 24 compares the difference between the measured relativecarrier phase and the predicted relative carrier phase to a thresholdvalue determine if a measurement anomaly is present. For example, thethreshold can be equal to a quarter of a characteristic wavelengthassociated with a carrier of the received signals. If the differenceexceeds the threshold, an anomaly flag can be triggered to indicate thata measurement anomaly is present. For example, a measurement anomaly canindicate a tracking error associated with one of the antennas or thepresence of fake signals as might be caused by spoofing or meaconing.

FIG. 2 illustrates one implementation of a global navigation satellitesystem (GNSS) 50 utilizing anomaly detection in accordance with anaspect of the present invention. In the illustrated implementation, aplurality of receivers 52-54 detect a navigation signal provided by oneor more GNSS satellites. At each receiver 52-54, the incoming satellitesignals are received by one or more elements on a multi-element GPSantenna 56-58 and converted into usable signals. For example, thereceivers 52-54 can include a multi-channel RF front-end thatdownconverts the received signal to baseband. The downconverted signalsare then digitized and provided to a plurality of signal extractors60-62. In the illustrated implementation, the signal extractors 60-62can include correlators configured to search the Doppler and delaycorrelation space for the strongest correlation energy indicating thepresence of an incoming signal. The correlators can be setup to providelocalized correlation values at points surrounding the acquired signalswhich then allows signal tracking. Once the signal is located by thecorrelators, range finding information is extracted from the signal,making it possible to determine a time of transit of the signal, andthus a psuedorange to the transmitter from each receiver 52-54.

The extracted range finding information can include a code pseudorange,which is the “distance” between the transmitter at some transmit timeand the receiver at some receive time. Because the transmit time and thereceive time are different, it is impossible to measure the true rangebetween the satellite and the receiver. A phase psuedorange is based onthe carrier phase of the signal and does not require the actualinformation being transmitted. In the illustrated implementation, thecarrier phase is used instead of the code or phase psuedorange. Todetermine the carrier phase (i.e., accumulated Doppler range), afractional beat phase of the received signal with a signal from a localoscillator having known properties can be measured and converted intothe range domain by scaling the measured beat with the wavelength.

A system control 70 includes a relative carrier phase (RCP) calculationcomponent 72 that determines a relative phase between the two receiversfrom the determined phase measurements. It will be appreciated that thesystem control 70 can be located at one of the receivers (e.g., 52),distributed among the plurality of receivers 52-54, or located remotely.The illustrated system 50 uses differenced phase processing to determinethe relative phase. Differenced phase processing generally usesmeasurements from two or more receivers at arbitrary positions to cancelcommon errors via carrier phase interferometry techniques. Theillustrated system 50 uses double differenced processing to form theinterferometric observations. In double difference processing, singledifferences are formed by determining differences between observationsfrom two separate receivers to a single satellite. Taking the differencebetween two single differences for a specific receiver pair gives thecarrier phase double difference, which can be used to determine therelative carrier phase between the pair of receivers.

The system control 70 can further include a relative carrier phasepredictor 74 that calculates a predicted relative carrier phase betweentwo receivers according to a known baseline between the receivers. Inone implementation, the predicted relative carrier phase is calculatedas a product of a line of sight matrix, representing a line of sightbetween the transmitters and at least one point on the baseline, and aposition vector representing the known baseline (e.g., a relativeposition of the two receivers). It will be appreciated that the line ofsight matrix generally represents a direction of one or moretransmitters from one or both of the receivers in the pair defining thebaseline. It will be appreciated that the baseline between two receiverscan be rigid and measured during a configuration of the system toprovide the known baseline. In the illustrated implementation, however,a flexible (i.e., variable) baseline is assumed, and the system control70 can be operatively connected to a baseline measurement component 76configured to dynamically measure the baseline. For example, thebaseline measurement component 76 can include a laser rangefinderconfigured to measure the distance between the two receivers,specifically, between the antennas associated with the receivers.

A calibration can be performed to determine the baseline vector and itsorientation in space within a body-referenced coordinate frame used byan inertial navigation system (INS) 78. The INS can be used tocontinuously provide the dynamic relationship between the body andnavigation frames and thus the ability to predict the relative carrierphase measurements. The coordinate frame transform can be accomplishedusing two direction cosines matrices. A first matrix provides a staticcalibration which relates a survey frame to the body frame, while thesecond matrix provides an on-the-fly conversion which relates the staticbody coordinate frame to the dynamic navigation frame, for example,using the relationship determined at the INS 78. Accordingly, a known ormeasured baseline in the body frame can be represented in the navigationframe to allow for prediction of the relative carrier phase.

It will be appreciated that the measured carrier phase can have a degreeof ambiguity, for example, in the integer number of wavelengths betweentwo phase measurements. This carrier phase ambiguity can be resolvedusing a known rigid baseline constraint. This involves surveying thebaseline length in the body frame and then transforming it from body tonavigation coordinates using external sensors. If the platform isnominally a two-dimensional platform such as a car or a train, only aheading is required from an external sensor such as a magnetometer. Fora three-dimensional platform, such as a boat or an aircraft, a leveledIMU can be added to estimate the roll and pitch of the platform. As longas the external sensors provide accurate enough orientation to predictthe rotated baseline within one-half of a wavelength, the ambiguity canbe deterministically resolved in a single epoch.

Consider, for example. a 2D rigid baseline of length one meter and amagnetometer with an accuracy of ±1 degree. For this case, themagnetometer can be used to predict the baseline in navigationcoordinates with an accuracy of 17.46 mm. A longer baseline amplifiesthe position prediction error for a given magnetometer error. For a twometer baseline and the same magnetometer, the position predictionaccuracy would be 34.91 mm. Both cases are within half of the 19 cm L1GPS wavelength.

Accordingly, a raw widelane quantity, WL_(raw), can be computed as:

$\begin{matrix}\begin{matrix}{{WL}_{raw} = {\lbrack {\frac{( {{SD}_{L\; 1} - {SD}_{L\; 1\; {key}}} }{\lambda_{L\; 1}} - \frac{( {{SD}_{L\; 2} - {SD}_{L\; 2\; {key}}} )}{\lambda_{L\; 2}}} \rbrack \cdot \lambda_{WL}}} \\{= {\lbrack {\frac{{DD}_{{L\; 1},{raw}}}{\lambda_{L\; 1}} - \frac{{DD}_{{L\; 2},{raw}}}{\lambda_{L\; 2}}} \rbrack \cdot \lambda_{WL}}}\end{matrix} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

where SD_(L1) is a single differenced phase measurement on the L1carrier, where SD_(L2) is a single differenced phase measurement on theL2 carrier, where DD_(L1, raw) is a raw double differenced phasemeasurement on the L1 carrier, where DD_(L2, raw), is a raw doubledifferenced phase measurement on the L2 carrier, λ_(L1) is a wavelengthof the L1 carrier, λ_(L2) is a wavelength of the L2 carrier, and λ_(WL)is the wide lane wavelength

The body-frame baseline and heading, DD_(est), can be parameterized as:

$\begin{matrix}\begin{matrix}{{DD}_{est} = {H \cdot b^{n}}} \\{= {H \cdot C_{b}^{n} \cdot b^{b}}} \\{= {{H\begin{bmatrix}{\cos \; \psi} & {\sin \; \psi} & 0 \\{{- \sin}\; \psi} & {\cos \; \psi} & 0 \\0 & 0 & 1\end{bmatrix}} \cdot b^{b}}} \\{= {H\begin{bmatrix}{{b_{x}^{b}\cos \; \psi} + {b_{y}^{b}\sin \; \psi}} \\{{{- b_{x}^{b}}\sin \; \psi} + {b_{y}^{b}\cos \; \psi}} \\b_{z}^{b}\end{bmatrix}}}\end{matrix} & {{Eq}.\mspace{14mu} 2}\end{matrix}$

where b^(n) is the baseline in a navigational frame, H is the heading,b^(b) is the baseline in a body frame, and C_(b) ^(n) is a rotationmatrix representing the alignment of the body in the navigation frame.

The narrowlane ambiguity, N_(WL), can be determined as a differencebetween the raw widelane ambiguity and the body frame baseline andheading, such that N_(WL)=round(WL_(raw)−DD_(est)). A final widelanequantity, WL, can be computed as WL_(raw)−N_(WL)*λ_(WL). A handover, HO,can be computed as a moving average filter, such thatHO=wavg(DD_(L1,raw)−WL), with an L1 ambiguity, N_(L1), computed asN_(L1)=round(HO/λ_(L1)). An L2 ambiguity is determined as a differencebetween the L1 ambiguity and the narrowlane ambiguity. From thesevalues, single frequency measurement, DD_(L1) and DD_(L2), can bedetermined as:

DD _(L1) =DD _(L1,raw) −N _(L1)·λ_(L1) and DD _(L2) =DD _(L2,raw) −N_(L2)·λ_(L2)

Once a carrier phase between at least two receivers has been measuredand a predicted relative carrier phase has been calculated, both valuesare provided to an anomaly detection component 80. The anomaly detectioncomponent 80 determines if a measurement anomaly is present from adifference between the predicted relative carrier phase and the measuredrelative carrier phase. In the illustrated implementation, the anomalydetection component 80 compares a difference between the measuredrelative carrier phase and the predicted relative carrier phase to athreshold value determine if a measurement anomaly is present. Forexample, the threshold can be equal to a quarter of a characteristicwavelength associated with a carrier of the received signals. If thedifference exceeds the threshold, an anomaly flag can be triggered toindicate that a measurement anomaly is present. For example, ameasurement anomaly can indicate a tracking error associated with one ofthe antennas or the presence of fake signals as might be caused byspoofing or meaconing.

Many interferometry systems have spatially or temporally separatedantennas (or elements) which could employ the techniques for anomalydetection described herein. For example, many anti-jam navigationsystems use a controlled reception pattern antenna or enhanced jammingprotection. FIG. 3 illustrates a controlled reception pattern antenna100 that could utilize an anomaly detection system in accordance with anaspect of the present invention. The controlled reception patternantenna 100 comprises a plurality of antenna nodes 102-108 maintained atconstant relative positions. Accordingly, respective baselines 110-113between a first antenna node 102 and neighboring antenna nodes 103-106are rigid, and can be measured, for example, during an antennaself-calibration survey. In the illustrated implementation, these rigidbaselines are measured in a body-referenced coordinate frame through astatic survey and must be dynamically converted to a navigationcoordinate frame to account for movement and rotation of the platformbefore they can be used for measurement prediction. Measurementanomalies can be detected for the CRPA when the phase measurements areprocessed individually from each antenna element, and the measurementsdo not match the predicted phase measurements.

Another example antenna arrangement 120 is shown in FIG. 4, in which theanomaly monitoring system monitors a rigid baseline 122 betweenredundant antennas 124 and 126 to validate nodal measurements which areconstituents of a flexible baseline (e.g., 128 or 130) between one ofthe redundant antennas (e.g., 124 or 126) and a third antenna 132.Essentially, the addition of a redundant antenna (e.g., 126) creates arigid baseline 122 with one of the antennas 124 in a pair of antennas124 and 132 needed for an application. This rigid baseline 122 allowsfor a more accurate prediction of the relative carrier phase, andaccordingly, for more accurate anomaly monitoring. Since many sources oferror would affect all local antennas, the more accurate anomalymonitoring over the rigid baseline 122 can be used to ensure theintegrity of the entire array 120. Redundant antennas might be used, forexample, on an airborne refueling platform to facilitate the use of ananomaly monitoring system as an input to an integrity monitor. A similarconfiguration could be used for very long baseline interferometry (VLBI)radars, in which the radar aperture is extended to include multipleantennas and the radar measurement is compared against a predictiondrawn from redundant antennas.

While the example of FIG. 2 utilizes multiple antennas and multiplereceivers, the same techniques can also be used temporally, with theantenna baseline formed at two or more discrete times from the samereceiver. The temporal baseline can be constrained by an externalobservation of displacement (e.g., inertial measurement unit orodometer) and thereby provide a means to predict the expected carrierphase measurement and form a detection metric.

In the examples of FIGS. 2-4, the anomaly detector can detectmeasurement errors whose magnitude is a small fraction of themeasurement wavelength as long as the anomaly is larger than themeasurement noise. Anomalies include events at the receiver-level, suchas tracking errors, or at the signal level, such as fake signals likespoofing or meaconing. This technique is not limited by the correctnessof ambiguity resolution, or error sources such as correlated noise,multipath, or cycle slips. In fact, an anomaly detection system inaccordance with an aspect of the present invention is sensitive enoughto detect these error sources.

In view of the foregoing structural and functional features describedabove, an example method will be better appreciated with reference toFIG. 5. While, for purposes of simplicity of explanation, the method ofFIG. 5 is shown and described as executing serially, it is to beunderstood and appreciated that the present invention is not limited bythe illustrated order, as some actions could, in other examples, occurin different orders from that shown and described herein or could occurconcurrently.

FIG. 5 illustrates a method 150 for monitoring carrier phase anomaliesin a range finding system in accordance with an aspect of the presentinvention. At 152, a relative carrier phase between first and secondantennas is predicted as a function of a relative position between thetwo antennas. It will be appreciated that the relative position can beknown from a calibration of the range finding system and simply storedas a parameter in a system control. Alternatively, the relative positionbetween the two antennas can be periodically determined, for example,via laser range finding or another appropriate means for tracking therelative position of two objects. In general, the relative positionbetween the two antennas is determined in a body-referenced coordinateframe associated with the range finding system and translated to anavigation coordinate frame via a coordinate transform. In oneimplementation, the relative carrier phase is predicted as a function ofthe relative position between the two antennas and a known position of aplurality of transmitters in the range finding system. For example, thepredicted relative carrier phase can be calculated as a product of aline of sight matrix, representing the relative position of theplurality of transmitters and at least one point associated with abaseline between the two antennas, and a vector representing therelative position of the two antennas.

At 154, a relative carrier phase between the first and second antennasis measured based upon at least one transmitted signal received at eachof the first and second antennas. In one implementation, a doubledifferenced carrier phase can be calculated for the two antennas usingsignals from two transmitters having known locations relative to theantennas. At 156, an anomaly detection metric is determined as afunction of a difference between the measured relative carrier phase andthe predicted relative carrier phase. In one implementation, the anomalydetection metric is a linear function of the difference between themeasured relative carrier phase and the predicted relative carrierphase, but it will be appreciated that, depending on the analysis meansused to detect the anomaly, that non-linear functions of this differencemay be useful.

At 158, it is determined if an anomaly is present according to theanomaly detection metric. In one implementation, an anomaly isdetermined to be present if the anomaly detection metric exceeds apredetermined threshold value, such as one-quarter of a wavelengthassociated with one of the at least one transmitted signal. In anotherimplementation, a rule-based expert system is provided with a timeseries of calculated anomaly detection metrics to determine a likelihoodthat an anomaly is present. Once a measurement anomaly is detected, itcan be flagged and reported to an operator to allow for appropriateadjustment to the range finding system.

The invention has been disclosed illustratively. Accordingly, theterminology employed throughout the disclosure should be read in anexemplary rather than a limiting manner. Although minor modifications ofthe invention will occur to those well versed in the art, it shall beunderstood that what is intended to be circumscribed within the scope ofthe patent warranted hereon are all such embodiments that reasonablyfall within the scope of the advancement to the art hereby contributed,and that that scope shall not be restricted, except in light of theappended claims and their equivalents.

Having described the invention, we claim:
 1. A method for monitoringcarrier phase anomalies in a range finding system comprising: predictinga relative carrier phase between first and second antenna locations as afunction of a relative position between the two antenna locations;measuring a relative carrier phase between the first and second antennalocations based upon at least one transmitted signal received at each ofthe first and second antenna locations; calculating an anomaly detectionmetric as a function of a difference between the measured relativecarrier phase and the predicted relative carrier phase; and determiningif an anomaly is present according to the anomaly detection metric. 2.The method of claim 1, wherein determining if an anomaly is presentaccording to the anomaly detection metric comprises determining if theanomaly detection metric exceeds a predetermined threshold value.
 3. Themethod of claim 2, wherein the predetermined threshold value isone-quarter of a wavelength associated with one of the at least onetransmitted signal.
 4. The method of claim 1, wherein determining if ananomaly is present according to the anomaly detection metric comprisesproviding a time series of calculated anomaly detection metrics to arule-based expert system.
 5. The method of claim 1, wherein predictingthe relative carrier phase between first and second antenna locationscomprises predicting the relative carrier phase as a function of therelative position between the two antenna locations and a known positionof a plurality of transmitters in the range finding system.
 6. Themethod of claim 5, wherein the predicting the relative carrier phasebetween first and second antenna locations comprises computing a productof a line of sight matrix, representing the relative position of theplurality of transmitters and at least one point associated with abaseline between the two antenna locations, and a vector representingthe relative position of the two antenna locations.
 7. The method ofclaim 1, further comprising periodically determining the relativeposition between the two antenna locations.
 8. The method of claim 1,further comprising: determining the relative position between the twoantenna locations in a body-referenced coordinate frame associated withthe range finding system; and transforming the determined relativeposition to a navigation coordinate frame via a coordinate transform. 9.A system comprising: a first antenna configured to receive a signal froma transmitter; a second antenna configured to receive the signal fromthe transmitter, the second antenna being separated from the firstantenna by a baseline; a signal processor configured to calculate ameasured relative carrier phase between the first antenna and the secondantenna according to the received signal; a relative carrier phaseestimator configured to estimate a predicted relative carrier phasebetween the first antenna and the second antenna according to thebaseline between the first antenna and the second antenna; and ananomaly detection component configured to determine if an anomaly ispresent according to an anomaly detection metric, the anomaly detectionmetric being determined as a function of a difference between themeasured relative carrier phase and the predicted relative carrierphase.
 10. The system of claim 9, wherein the baseline between the firstantenna and the second antenna is constant, the relative carrier phaseestimator storing a position vector representing the baseline.
 11. Thesystem of claim 9, further comprising a laser rangefinder to dynamicallymeasure the baseline between the first antenna and the second antenna.12. The system of claim 9, further comprising an inertial measurementunit configured to monitor rotation of a platform associated with atleast one of the first antenna and the second antenna, the relativecarrier phase estimator being further configured to translate thebaseline between the first antenna and the second antenna from acoordinate frame associated with the platform to a navigation coordinateframe according to the monitored rotation.
 13. The system of claim 9,wherein the relative carrier phase estimator estimates the predictedrelative carrier phase between first and second antennas as a functionof the baseline between the two antennas and a known position of aplurality of transmitters associated with the system.
 14. The system ofclaim 13, wherein the relative carrier phase estimator is configured tocompute a product of a line of sight matrix, representing the relativeposition of the plurality of transmitters and at least one pointassociated with a baseline between the two antennas, and a vectorrepresenting the relative position of the two antennas.
 15. The systemof claim 9, further comprising a third antenna having a flexiblebaseline with each of the first antenna and the second antenna, themetric comparison component determining if an anomaly is present betweenthe third antenna and one of the first antenna and the second antennaaccording to the anomaly detection metric.
 16. The system of claim 15,wherein each of the first antenna, the second antenna, and the thirdantenna are implemented on an airborne refueling platform, an output ofthe anomaly detection component being provided to an integrity monitorassociated with the airborne refueling platform.
 17. The system of claim9, wherein the first antenna and the second antenna are first and secondnodes of a plurality of antenna nodes comprising a controlled receptionpattern antenna array.
 18. The system of claim 9, wherein the anomalydetection component comprises a rule-based expert system.
 19. A globalnavigation satellite system (GNSS) comprising: a first receiverconfigured to receive signals from a plurality of GNSS satellites; asecond receiver configured to receive signals from the plurality of GNSSsatellites, respective antennas of the first and second receivers beingseparated by a known baseline; a signal processor configured tocalculate a double differenced carrier phase between the first receiverand the second receiver according to the received GNSS satellitesignals; a relative carrier phase estimator configured to estimate apredicted relative carrier phase between the first receiver and thesecond receiver according to the baseline between the first antenna andthe second antenna; and an anomaly detection component configured todetermine that an anomaly is present if a difference between themeasured relative carrier phase and the predicted relative carrier phaseexceeds a predetermined threshold value.
 20. The GNSS system of claim19, wherein the predetermined threshold value is one-quarter of awavelength associated with a GNSS carrier.
 21. The GNSS system of claim19, wherein the plurality of GNSS satellites is a first GNSSconstellation and the anomaly detection component is further configuredto use observations from the first GNSS constellation to validate atleast one observation from a second GNSS constellation.